Customary International Law and State Patterns of Cooperation in the Use of Force in Response to Terrorism

State practice and patterns of cooperation over the last 45 years have led to the development of rules of customary international law governing the use of force, in anticipatory self-defense, against terrorists and rogue state collaborators. Although the earlier general rules may have prohibited states from using force except in anticipation of an imminent attack, in more recent practice, the imminence standard has changed. States have initiated and cooperated in the use of force in self-defense to instances in which the possibility of an attack is not imminent, but merely expected. These actions are based on an assessment of the following factors: (1) the protection of nationals; (2) the probability of an attack; (3) the magnitude of potential harm; (4) the need to disrupt terrorist planning and activities; and (5) the need to eliminate safe havens. These rules have emerged with considerable cooperation from the states most actively engaged by the treat of terrorism.